Seed crystal for epitaxial growth of single-crystal calcium fluoride

ABSTRACT

A nucleant seed for epitaxial growth of single-crystal CaF 2  includes SrF 2 . In some embodiments, YF 3 , LaF 3 , or rare-earth fluoride is substituted into the SrF 2  structure.

This application is a continuation of U.S. patent application Ser. No.09/818,160, filed Mar. 27, 2001, now U.S. Pat. No. 6,451,111 and claimsthe priority of said application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to processes for producing CaF₂crystals. More specifically, the invention relates to a nucleant seedfor epitaxial growth of single-crystal CaF₂.

2. Background Art

Single-crystal CaF₂ is commonly grown using the Bridgman-Stockbargercrystal growth process. For epitaxial growth of CaF₂, the processstarts, as illustrated in FIG. 1, with a seed crystal 2 made of CaF₂ andhaving the desired crystallographic orientation. For deep-ultravioletmicrolithography applications, for example, the desired crystallographicorientation is <111>, i.e., cubic (octahedral or cubic forms) crystalstructure. The seed crystal 2 is placed at the base of a crucible 4. Astarting material 6 comprising CaF₂ powder (or beads) is placed in thecrucible 4, on top of the seed crystal 2. The crucible 4 is then placedin a vertical furnace 8 and heated to a temperature sufficient to meltthe starting material 6. To prevent oxidation of the starting material 6and the components of the furnace 8, the furnace 8 is typicallymaintained under vacuum and/or the process is carried out in an inertatmosphere.

After melting the starting material 6, the crucible 4 is moveddownwardly at a predetermined rate (typically 0.3 to 5 mm/h), from a hotzone 10 into a cold zone 12. An insulating barrier 14 separates the hotzone 10 from the cold zone 12. FIG. 2 shows a typical temperaturedistribution along the vertical axis of the furnace (8 in FIG. 1). Asingle crystal of CaF₂ forms on the seed crystal (2 in FIG. 1) when themolten material reaches the zone 12 in which the furnace temperature isbelow the melting point of CaF₂. The CaF₂ crystal front propagatesinside the crucible 4, within the material 6, as long as the crucible 4is caused to move downwardly. The CaF₂ crystal conforms to thecrystallographic orientation of the seed crystal 2 as it propagatesinside the crucible 4.

To enhance the optical properties of the CaF₂ crystal, a scavenger istypically added to the starting material 6 to remove oxygen and hydroxylions. These impurities have been known to reduce transmission in thedeep-ultraviolet region. The most common scavenger used is PbF₂. PbF₂ issolid and can be added directly to the starting material 6. Typically, aspecific amount of PbF₂, typically 1 to 2% by weight, is mixed into thestarting material 6. The mixture is then gradually heated toapproximately 800° C. to 900° C., at which point PbF₂ reacts with thestarting material 6 to form PbO. After the reaction is complete, themore volatile PbO is evaporated from the mixture by heating the mixtureto the melting point of CaF₂ or higher. In an attempt to remove as muchof the PbO as possible through volatization, the CaF₂ melt may becomeoverheated and cause the seed crystal 2, which is also made of CaF₂, tocompletely melt and lose its crystallographic orientation.

SUMMARY OF THE INVENTION

In one aspect, the invention relates to a nucleant seed for epitaxialgrowth of single-crystal CaF₂ which comprises SrF₂. In some embodiments,a second fluoride is substituted in the SrF₂ structure, the secondfluoride being selected from the group consisting of YF₃, LaF₃,rare-earth fluoride, and combinations thereof. In some embodiments, therare-earth fluoride comprises one selected from the group consisting ofYF₃, LaF₃, CeF₃, NdF₃, PrF₃, DyF₃, SmF₃, EuF₃, TbF₃, and GdF₃.

Other aspects and advantages of the invention will be apparent from thefollowing description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a Bridgman-Stockbarger crystal growth process.

FIG. 2 shows a temperature distribution along a vertical axis of thefurnace shown in FIG. 1.

FIG. 3 shows a phase diagram for SrF₂ and LaF₃.

DETAILED DESCRIPTION

Embodiments of the invention provide a seed crystal for use in growingoriented, single-crystal CaF₂. The seed crystal is structurally similarto CaF₂ but has a higher melting point than CaF₂. In one embodiment ofthe invention, the seed crystal comprises SrF₂. There iscrystallographic disregistry between SrF₂ and CaF₂, but this disregistryis well within the accepted values for effective nucleation of CaF₂ bySrF₂. In other embodiments of the invention, the seed crystal comprisesa solid solution of SrF₂ and a fluoride such as LaF₃, YF₃, rare-earthfluorides, or combinations thereof. Examples of rare-earth fluoridessuitable for use in the invention include, but are not limited to, CeF₃,NdF₃, PrF₃, DyF₃, SmF₃, EuF₃, TbF₃, and GdF₃. The effect of the fluoridesubstituted in the SrF₂ structure is to further increase the meltingpoint of the seed crystal.

CaF₂ melts around 1415° C. The only other known fluoride phase with thesame structure as CaF₂ and which has a melting point higher than CaF₂ isthe strontium analog SrF₂. The melting point of this phase of SrF₂ isnear 1455° C., about 40° C. higher than CaF₂. When pure SrF₂ is used asa seed crystal for the growth of CaF₂ crystal, the starting material (6in FIG. 1) can be heated to much higher temperatures than the meltingpoint of CaF₂ without melting the SrF₂ seed crystal. Of course, givenenough time, that time being dependent upon kinetic factors, anessentially infinite reservoir of CaF₂ liquid would eventually dissolveeven a refractory seed like SrF₂ or SrF₂-LaF₃ solid solution, but thetime of survival will be longer than with a metable CaF₂ seed.

When a scavenger such as PbF₂ is added to the starting material (6 inFIG. 1), maximum removal of the by-products of the scavenging processcan be removed via volatization without melting the SrF₂ seed crystal.Even if the SrF₂ seed crystal succumbs to dissolution in overheated CaF₂melt, the process will be much slower than for melting or dissolution ofa CaF₂ seed. Thus, removal of the by-products of the scavenging processcan be completed before the seed crystal completely melts and loses itscrystallographic orientation.

The melting point of the SrF₂ seed crystal can be increased by adding afluoride, e.g., YF₃, LaF₃, rare-earth fluoride, or combinations thereof,to SrF₂. The strontium analog SrF₂ phase forms considerable solidsolution, often up to 50 mole %, with YF₃, LaF₃, and rare-earthfluorides such as CeF₃, NdF₃, PrF₃, DyF₃, SmF₃, EuF₃, TbF₃, and GdF₃.The solid solution is formed by mixing molten SrF₂ with the fluoride andthen cooling the mixture. The solid solution can be made to have adesired crystallographic orientation by cooling the mixture at a certaintemperature. The fluoride gets substituted in the SrF₂ structure. Theeffect of these substitutions is usually to further increase the meltingpoint of the SrF₂-CaF₂ solid solution. The melting point can beincreased by 50° C. to 100° C. with LaF₃ and rare-earth substitutions inthe 10 to 30 mole % range. The following table shows the melting pointachieved by various fluoride substitutions in SrF₂.

TABLE 1 Melting Point for Solid Solutions of SrF₂ and rare-earthfluorides Seed Crystal Substitution in mole % Melting Point (° C.0) PureSrF₂ 1455 SrF₂—YF₃ 11 1460 SrF₂—LaF₃ 30 1550 SrF₂—CeF₃ 29 1550 SrF₂—NdF₃25 1535 SrF₂—PrF₃ 30 1540 SrF₂—DyF₃ 12 1490 SrF₂—SmF₃ 21 1525 SrF₂—EuF₃20 1510 SrF₂—TbF₃ 15 1500 SrF₂—GdF₃ 16 1520

The solid solutions shown in Table 1 above are expected to be effectiveas nucleant seeds for the epitaxial growth of CaF₂. Some small-ionrare-earths like Ho, Er, Yb, and Lu either lowered or did not increasethe melting point of the seed crystal. It should be noted that LaF₃,YF₃, or rare-earth fluorides themselves are not appropriate as seedcrystals because they are structurally dissimilar to CaF₂. Furthermore,the structure of the solid solution will become dissimilar to CaF₂ iftoo much fluoride is mixed into SrF₂. The amount of fluoride to be mixedinto SrF₂ can be deduced from appropriate phase diagrams. See, forexample, L. P. Cook and H. F. McMurdie, Eds., “Phase Diagrams forCeramists,” vol. V11, FIGS. 7581-7987, American Ceramic Society, 1989.FIG. 3 shows a phase diagram for SrF₂ and LaF₃. The phase diagram showsthat SrF₂ forms solid solution of up to approximately 47 mole % withLaF₃. The highest melting point of the SrF₂-LaF₃ solid solution occurswhen LaF₃ substitution is about 30 mole %.

The invention provides general advantages. By using structurally similarbut more refractory nucleant seed for growing the single-crystal CaF₂,maximum removal of the by-product of the scavenging process can beremoved without completely melting the seed crystal.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

What is claimed is:
 1. A nucleant seed for epitaxial growth of asingle-crystal CaF₂ comprising SrF₂.
 2. A nucleant seed for epitaxialgrowth of a single crystal CaF₂ comprising a fluoride substituted in theSrF₂ structure, the fluoride selected from the group consisting of YF₃,LaF₃, rare-earth fluoride, and combinations thereof.
 3. The nucleantseed of claim 2, wherein the rare-earth fluoride comprises one selectedfrom the group consisting of CeF₃, NdF₃, PrF₃, DyF₃, SmF₃, EuF₃, TbF₃,and GdF₃.
 4. The nucleant seed of claim 3, wherein the fluoridesubstitutions in the SrF₂ structure is in a range from 10 to 30 mole %.5. A process for producing a single-crystal CaF₂ from a melt,comprising: contacting the melt with a seed comprising SrF₂; and movingthe melt at a rate in the range of 0.3 to 5 mm/h through athermally-graded zone so that the single-crystal CaF₂ is grown on theseed.
 6. The process of claim 5, wherein the seed further comprises afluoride substituted in the SrF₂ structure, the fluoride selected fromthe group consisting of YF₃, LaF₃, rare-earth fluoride, and combinationsthereof.
 7. The process of claim 6, wherein the rare-earth fluoridecomprises one selected from the group consisting of YF₃, LaF₃, CaF₃,NdF₃, PrF₃, DyF₃, SmF₃, EuF₃, TbF₃, and GdF₃.
 8. The process of claim 7,wherein the rare-earth substitutions the SrF₂ is in a range from 10 to30 mole %.
 9. A process for producing a single-crystal CaF₁ from a melt,comprising: contacting the melt with a seed having a composition SrF₂-X,where X is selected from the group consisting of YF₃, LaF₃, rare-earthfluoride, and combinations thereof; and moving the melt at a rate in therange of 0.3 to 5 mm/h through a thermally-graded zone so that the CaF₂crystal is grown on the seed.
 10. The process of claim 9, wherein therare-earth fluoride comprises one selected from the group consisting ofYF₃, LaF₃, CeF₃, NdF₃, PrF₃, DyF₃, SmF₃, EuF₃, TbF₃, and GdF₃.
 11. Theprocess of claim 10, wherein the rare-earth substitution in the SrF₂ isin a range from 10 to 30 mole %.